The sequel to the movie that raised Robert Downey jnr's star stocks to an all-time-high, arrives just two years after the original.

That was film which proved that not all comic book-derived superheroes had to be dark knights.

The first was fun, mainly because of Downey's sharp-witted turn as Tony Stark, a weapons tech-tycoon whose invincible fighting and flying suit, patched into his own nuclear pacemaker, rendered him a true Man of Steel.

The second is bigger in every way. It comes with more sidekicks, villains, stars, cameos, locations, storylines and scenes of metal manbots panelbeating each other into submission.

But somehow all that makes this less fun than the first.

Part of that is a story engagement problem. You know this is going to end up in an inevitable demolition derby.

But by the time that swings round, we've mislaid the reason for the fight.

Revenge? Market share? So that Stark can find a safer alternative energy source for that glowing thingamy in his chest keeping him alive? To get Scarlett Johansson back into the catsuit just one more time?

All of the above probably, but none provide any good reasons to get wrapped up in its convolutions and occasional nonsensical scene. Some of those involve an eye-patched Samuel L. Jackson who turns up, unintroduced, halfway through, as Nick Fury, a character undoubtedly highly significant to Iron Man's Marvel Comic family tree - and an inevitable franchise opening of The Avengers - but just confusing and annoying here.

This one picks up with Stark, who had outed himself as Iron Man at the end of the first chapter, enjoying his role as global saviour - as he says, the man "who privatised world peace".

But Mickey Rourke's mad Ruskie physicist is out to avenge his father's fate at the hands of Stark's late dad. He's a dab hand with a soldering iron and is soon making a nuisance of himself in his own exo-skeleton.

Oh, and Stark has to fix the biophysics flaws in his masterpiece and find time to fix the chemistry with Gwyneth Paltrow's Pepper Potts, his former PA, who he appoints chief executive.

With all that going on, it sure feels like a movie with a lot of explaining to do and story sidelines to cross-cut before another big action scene rolls around.

Some of those are pretty good. And Downey's corporate-megalomaniac-turned-superman is, again, quite an act, especially in a movie which seems to play on the actor's past problems - like a drunken party scene also starring, as himself, the late DJ AM, who died last year from an accidental overdose.

Downey brings plenty of that high-voltage personality to the role again, and fellow back-from-the-brink star Rourke sure makes for an oddball villain.

But while they hold the attention, the movie behind them feels like one distraction after another.